1. Field of the Invention
The present invention relates to an LED drive circuit and an LED illumination light.
2. Description of the Related Art
An LED (Light Emitting Diode) features of low power consumption, long life and the like, and is finding its wide applications in not only display apparatuses but also in illumination lights and the like. Here, in an illumination light, to obtain a desired illuminance, there are many cases where a plurality of LEDs are used (e.g., JP-A-2006-319172).
A general illumination light often uses a commercial 100 VAC power supply, and considering a case where an LED illumination light is used instead of a general illumination light such as an incandescent lamp and the like, it is desirable that like a general illumination light, an LED illumination light also is structured to use a commercial 100 VAC power supply.
Here, in a case of performing light adjustment control of an incandescent lamp, a phase control type light adjuster (generally, called an incandescent lamp controller) is used (e.g., JP-A-2005-26142), which is capable of easily performing light adjustment control of electricity supply to the incandescent lamp by means of a volume device only by turning on a switching device (generally, a thyristor device, a TRIAC) at a phase angle of an a.c. power supply voltage. Also in a case of performing light adjustment of an incandescent lamp by means of a phase control type light adjuster, it is known that when the light adjuster is connected with an incandescent lamp that has a small wattage, flickering and blinking occur and normal light adjustment is impossible.
In a case of performing light adjustment control of an LED illumination light that uses an a.c. power supply, like in the case of performing light adjustment control of an incandescent lamp, it is desired to use a phase control type light adjuster. Here, FIG. 15 and FIG. 16 show conventional examples of LED illumination systems capable of performing light adjustment control of an LED illumination light that uses an a.c. power supply.
A conventional LED illumination system shown in FIG. 15 has: a phase control type light adjuster 200; an LED drive circuit 300; and an LED load 400 that includes a plurality of LEDs. The LED drive circuit 300 includes a full-wave rectifier 1 and a current limiting portion 2. Between an a.c. power supply 100 and the current limiting portion 2, the phase control type light adjuster 200 is connected in series. When a knob (not shown) of a semi-fixed resistor Rvar is set to a position, the phase control type light adjuster 200 turns on a TRIAC Tri at a power supply phase angle that corresponds to the set position. Further, in the phase control type light adjuster 200, a noise prevention circuit including a capacitor CLC and an inductance coil LLC is composed to reduce terminal noise that is fed back from the phase control type light adjuster 200 to a power supply line.
Besides, a conventional LED illumination system shown in FIG. 16 has: the phase control type light adjuster 200; an LED drive circuit 500; and the LED load 400. The LED drive circuit 500 has the full-wave rectifier 1 and a current control portion 3. The current control portion 3 has: a switching device SW1; a coil L2; a diode D1; a capacitor C4; a current detection resistor R2; and a switching control circuit 4. The switching control circuit 4 detects an effective value of an output voltage V2 from the full-wave rectifier 1 and a value of a current flowing in the current detection resistor R2 that is connected with a source of the switching device SW1; controls on/off of the switching device SW1; the and controls a current flowing in the switching SW1 to be a constant current. In accordance with a phase angle controlled by the phase control type light adjuster 200, it is possible to change a magnitude of a current flowing in the LED load 400 and becomes possible to perform light adjustment of the LED load 400.
Besides, FIG. 17 shows a conventional incandescent lamp illumination system that performs light adjustment of an incandescent lamp 5 by means of the phase control type light adjuster 200. Besides, FIG. 18 shows voltage and current waveforms at some portions of the conventional incandescent lamp illumination system shown in FIG. 17. The TRIAC Tri is turned on, whereby a voltage V3 across both ends of the incandescent lamp 5 rises and a current I1 begins to flow in the incandescent lamp. And, the on state of the TRIAC Tri is kept until the a.c. power supply voltage V1 comes close to 0 V and the current flowing in the TRIAC Tri becomes equal to or smaller than a holding current.
When the TRIAC Tri in the phase control type light adjuster 200 is turned on, energy stored in the capacitor CLC flows into the coil LLC and a resonance phenomenon occurs. In case of a load such as the incandescent lamp 5 (FIG. 17) and the like that needs much current, even if a current is vibrated, the TRIAC is not turned off. However, in a case of a small load like the LED load 400 (FIG. 15, FIG. 16), the current flowing in the TRIAC Tri becomes equal to or smaller than the holding current (e.g., about 10 mA) and the TRIAC Tri is likely to be turned off. At this time, because of the following two phenomena, flickering of the LED load 400 occurs.
First, after the TRIAC Tri is turned off because of the resonance phenomenon and the like, a trigger voltage is applied again to the TRIAC Tri, whereby the TRIAC Tri is turned on again within an identical a.c. half period (within 10 ms of half period in a case of 50 Hz). At this time, the timing the TRIAC Tri is turned on does not stabilize at every a.c. half period and the energy supplied to the LED load 400 does not stabilize, whereby the flickering of the LED load 400 occurs.
Second, like in the case where the incandescent lamp 5 is connected with the phase control type light adjuster 200 (FIG. 17), if the on state of the TRIAC Tri is kept until the a.c. power supply voltage V1 becomes nearly 0 V, the capacitor CLC is not charged at the timing the next a.c. half period begins. However, in a case where the TRIAC Tri is turned off when the a.c. power supply voltage V1 is a high voltage (e.g., 50 V in 100 VAC), the capacitor CLC is charged because of currents flowing in the LED load 400, the LED drive circuits 300 and 500; accordingly, in the next a.c. half period, the phase angle at which the TRIAC Tr1 is turned on deviates. The charge amount in the capacitor CLC changes, whereby the phase angle at which the TRIAC Tri is turned on changes and the flickering of the LED load 400 occurs.
Here, an example of the conventional LED drive circuit is disclosed in JP-A-2006-319172; this LED drive circuit has a resistor and a capacitor in an output stage. However, this resistor is intended to prevent a rush current and the capacitor is intended to remove noise, but is not intended to alleviate the resonance phenomenon of the phase control type light adjuster.